Role of the twin boundary for the occurrence of peak effect in Y-Ba-Cu-O superconducting bulks melt grown under varied oxygen pressure

A novel melt-growth technique, i.e. melt growth under varied oxygen pressure (MGVOP) in which the oxygen partial pressure is increased to solidify the initial melt, has been employed to prepare melt-textured superconducting bulks of the Y-Ba-Cu-O system. The obtained textured bulk samples consisted of two phases, i.e. superconducting YBa2Cu3O7-delta and non-superconducting Y2BaCuO5 (Y-211). The critical current density (J(c)) of the samples reached as high as 40 kA cm(-2) (at 65 K) at a high magnetic field of H-a = 3 T to exhibit a peak in the J(c) versus H-a relation, i.e. a peak effect. To elucidate the causes of the peak effect and the high values of J(c) at high magnetic fields, submicron structures of the MGVOP-processed samples were examined by transmission electron microscopy (TEM), and the distributions of both Y-211 particles and twin boundaries were studied. In order to collect sufficient data for statistical analyses, specimens for 'large-area TEM observations' were prepared using a focused ion beam thinning technique such that the TEM-observable area of the specimen was as wide as 20 x 20 mu m(2). Statistical analyses of the TEM images yielded the volume fraction of Y-211 phase, the average size of Y-211 grains and the inter-twin-boundary distance for each sample. It was concluded that a sample that had a higher twin-boundary density possessed a higher J(c) value at the peak in the J(c) versus H-a characteristics, while fine Y-211 particles were likely to contribute to the J(c) value only at low magnetic fields.